Method for reducing braking distance

ABSTRACT

The invention concerns a process for shortening the braking distance of a vehicle equipped with a brake servo unit which during a standard braking action is only triggered by the brake pedal pressure caused by the driver and in case of a critical driving situation is so triggered by a control device that in comparison to the standard braking action an increased amplification factor is set at the brake servo unit. The invention distinguishes itself by the fact that a danger potential is determined, stating the probability that the vehicle to be braked will be involved in an accident and that the increase of the amplification factor is controlled in accordance with the danger potential.

TECHNICAL FIELD

The invention generally relates to brake systems and more particularlyrelates to a process for shortening the braking distance of a vehicle.

BACKGROUND OF THE INVENTION

A process for shortening the braking distance is well-known from DE 8911 963 U1 where on the basis of two present signals a pre-braking actionis started for a duration of approx. 0.5 s. At the same time the firstsignal is triggered by the driver activating a switch. This switch is soarranged that it is either operated by the left foot or one hand and incase of the manual operation the switch is preferably so arranged thatthe hand does not have to be removed from the steering wheel. The secondsignal is triggered by recording the speed the driver uses to remove hisfoot from the accelerator. If this speed is above a certain thresholdvalue pre-braking is automatically started.

In case of this well-known process it is disadvantageous that the drivermust carry out a control function that is unusual when driving avehicle, meaning he/she has to operate a switch with the left foot andone hand, respectively.

Therefore a process is proposed in the genus forming DE 40 28 290 C1where a larger brake pressure is automatically created than the oneresulting from the position of the brake pedal when the speed the brakepedal is operated with exceeds a predetermined value. This is the onlycriterion for triggering the automatic braking action. In comparison tothe process well-known from DE 89 11 963 U1 this process is advantageoussince the driver of a vehicle equipped with a so-called brake assistanttriggering an automatic brake action does not have to specificallyadjust his/her behavior to it, but is able to operate the individualoperational controls, in particular the brake pedal, in the usual way.

Nevertheless, a panic situation is recognized and a brake action carriedout that correspondingly decelerates faster.

This process has proved its worth in panic situations since the brakingdistance can be shortened and an accident avoided.

In this process a maximum brake pressure is created by the brake servounit after the automatic brake action has been triggered in order toshorten the braking distance to the maximum. The automatic brake actionis ended when the brake pressure applied to the brake pedal decreases.Since the brake servo unit is automatically activated, the brake pedalpressure is able to reduce itself automatically since the pedalresistance decreases. As a result an unintentional deactivation of theautomatic brake action may be caused. Therefore the automatic brakeaction can be ended without the driver of the vehicle wanting orintending it.

On the other hand the automatic brake action may be triggeredunintentionally by a driver reacting quickly to a change in the drivingsituation causing a maximum of the vehicle's deceleration without thisbeing necessary or desired. In certain driving situations such asurprising, unintentionally strong braking may result in an disturbanceof the traffic flow, particularly in case of dense lines of traffic, orit may even case an accident with a following vehicle.

Although it is simple for the driver of a corresponding vehicle toopoperate this well-known process for shortening the braking distance inhis usual way, it may, however, in certain situations result in anunintentionally too low or too high a deceleration of the vehicle. Thusthe driver does not always have the complete control of his/her vehicle.

Brake servo units suitable for such processes to shorten the brakingdistance that can be controlled either mechanically by means of ancontrol bar or electrically by means of a solenoid are well-known fromDE 43 24 205 A1 and DE 195 48 705 A1.

The invention is based upon the task to create a process for shorteningthe braking distance for which the driver of a vehicle does not have tochange his behavior, yet is carrying out a suitable deceleration of thevehicle in all driving situations and which enables the driver tomaintain safe control of his/her vehicle even during a braking actiondecelerating more strongly.

According to the process in accordance with the present invention adanger potential is determined showing the probability that the vehicleto be braked will be involved in an accident. In accordance with thedanger potential the amplification factor of the brake servo unit isincreased in a dangerous situation.

Thus the process in accordance with the invention distinguishes itselffrom the one well-known out of DE 40 28 290 C1 essentially by the factthat during an automatic brake action there is no need for a maximumdeceleration but that the deceleration may take on variable values thatare lower for a lower danger potential than for a higher one. Thistriggers a brake action adjusted to the driving situation that can beconsiderably better controlled by the driver than a very sudden start ofa maximum deceleration.

The inventors of the present invention call a device controlling aprocess in accordance with the invention a so-called “analogous brakeassistant”. This should give expression to the fact that the process inaccordance with the invention as a rule brakes with continuouslyadjustable, variable deceleration values whilst the process known fromDE 40 28 290 C1 only switches over “digitally” between a mechanicallycontrolled braking and an automatic full brake application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 the function for the calculation of a drivers danger potentialΔ_(da).

FIG. 2 two vehicles following each other as an illustration of therelevant physical variables.

FIG. 3 each of them functions for the calculation of the to 6 drivingsituation danger potential Δ_(ds).

FIG. 4 is a graph of danger potential versus deceleration.

FIGS. 5 and 6 are graphs showing driving situation danger potential as afunction of a_(acve) and a_(acva).

FIG. 7 a first embodiment example of a process in accordance with theinvention in a block schematic diagram.

FIG. 8 a function showing the connection between the amplificationfactor Λ and the danger potential Δ.

FIG. 9 a modification of the first embodiment example of a process inaccordance with the invention in a block schematic diagram.

FIG. 10 a second embodiment example of a process in accordance with theinvention in a block schematic diagram.

FIG. 11 a modification of the second embodiment example of a process inaccordance with the invention in a block schematic diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process in accordance with the invention for shortening the brakingdistance triggers a brake servo unit as it is well-known, for example,from DE 195 48 705 A1. This brake servo unit can be directly andmechanically controlled at a control bar; then the brake pedal pressureF_(ped) put on the brake pedal is transferred to the control bar. Inaddition, this brake servo unit can also be electrically controlled by asolenoid in order to increase the brake pressure put, for example, onthe individual brake elements in danger situations.

In order to trigger the brake sensor unit electrically an electroniccontrol device is provided that is connected to various sensors in orderto carry out the process in accordance with the invention for shorteningthe braking distance of a vehicle.

Under normal driving conditions the process for shortening the brakingdistance works in a manner well-known as such; then the brake servo unitis triggered only mechanically by operating the control bar by means ofthe brake pedal. In critical driving situations the brake servo unit istriggered in such a way that, in comparison to an amplification factorΛ₀ available during a purely mechanical operation of the brake servounit an increased amplification factor Λ is set at the brake servo unitby an electrical activation of the brake servo unit. For this purpose adanger potential Δ is determined in accordance with the inventioncorresponding to the accident risk the vehicle to be braked is in.

Such an accident risk is not a physical variable and therefore it cannotbe directly measured. In the mathematical meaning the danger potential Δis rather a probability and therefore takes on values between 0 (noaccident risk) and 1 (acute accident risk, emergency situation). It goeswithout saying that the danger potential Δ can also be scaled in adifferent range of numbers, for example from 0 to 100. The followingdescription is based on a range of numbers from 0 to 1 for the dangerpotential Δ. It is essential for the invention that the danger potentialis able to take on several variables and does not only distinguishbetween two situations such as the existence of an accident risk and thenon-existence of an accident risk.

The danger potential is estimated from available physical variablesessentially based upon the confirmation of the brake signal by thedriver and the prevailing danger situation the vehicle to be braked isin and from statistical figures drawn from past experience such as themean reaction time.

A distinction is made between the driver danger potential Δ_(da)—basedon the data determined whilst the brake is activated—and the drivingsituation danger potential Λ_(ds)—based on the data determined out ofthe driving situation.

A driver recognizing a danger situation activates the individual controlelements of the vehicle in order to avoid an accident in acharacteristic way. He/she will, for example, very quickly take his/herright foot off the accelerator and activate the brake pedal. Inaddition, fast steering movements are possible that do not normallyoccur. Therefore the following physical variables can be used in orderto determine the driver danger potential Δ_(da):

the speed used to remove the foot from the accelerator,

the pedal changing time the driver needs to move from the accelerator tothe brake pedal,

the brake pedal travel,

the brake pedal speed,

the brake pedal force F_(ped),

the change in the brake pedal force F_(ped) and/or

the change in the steering angle.

On the basis of these measurable parameters the danger potential Δ_(da)can be estimated; at the same time the estimate may be based on a singleone of these parameters or on a combination of several parameters. Theabove parameters are not an exhaustive listing, on the contrary similarascertainable parameters may also be used to determine the dangerpotential, such as the force applied to the control bar of the brakeservo unit that is proportional to the brake pedal force F_(ped) or thetravel s_(mem) of a diaphragm provided within the brake servo unit andtheir differentiations in time that correspond to the travel of theaccelerator and its change, respectively, etc.

When using the brake pedal travel or any variable corresponding to it inorder to determine the danger potential, it shall be taken intoconsideration that with the start of the electrical control of the brakeservo unit the control bar of the brake servo unit and with it the pedalare automatically activated and therefore the brake pedal travel alonemay no longer serve for an objectively valid estimate. In addition, ithas been proved that the brake pedal is often activated with high speedbut only for a short time and with low force. In such a case the highpedal speed is no valid indicator for a danger potential. When the pedalspeed is used as an indicator for the danger potential, it is preferablycombined with another parameter, such as the brake pedal force or thebrake pedal travel. Since it is expensive to employ sensors for thedirect measurement of brake pedal travel, it is therefore expedient touse the shifting of a diaphragm s_(mem) in the brake servo unitcorresponding to the brake pedal travel as the indicator for the dangerpotential. As a result the output$P = {F_{ped} \cdot {\overset{.}{s}}_{mem}}$

can be used as an easily measurable variable realistically estimatingthe danger potential.

Other parameters for determining the driver danger potential Δ_(da) maybe, for example${s_{mem} \cdot {\overset{.}{s}}_{mem}},{{\overset{.}{F}}_{ped} \cdot s_{mem}},{{{\overset{.}{F}}_{ped} \cdot F_{ped}}\quad {or}\quad {{\overset{.}{F}}_{ped} \cdot {\overset{.}{s}}_{mem} \cdot}}$

Additionally or alternatively the steering angle and the acceleratortravel and their differentiations in time may be used.

It is also possible to use a combination of parameters with$\Delta_{da} = {\sum\limits_{i = 1}^{n}\quad {{{Parameter}_{i}} \cdot g_{i}}}$

with g_(i) representing a weighting factor for the respective parameter_(i).

In addition, a fault identification may be carried out by a comparisonof the pedal force with the pedal travel or diaphragm travel or thepressure built up in the brake servo unit or by comparing the diaphragmtravel with the pressure built up in the brake servo unit. Therefore asensor configuration with a pressure sensor and a force sensor ispreferred for the process in accordance with the invention because itcreates a solution with fault identification at a reasonable price.

In lieu of the control actions carried out by the driver the physicalconditions of the driver, such as his/her heart rhythm, eye movements,etc. may on principle also be used to determine a danger situation. Ifthese parameters are ascertainable with sensors working without contact,they may obtain a practical significance.

An embodiment example for determining the driver danger potential Δ_(da)is described below; at the same time the driver danger potential Δ_(da)is determined out of the change in the brake pedal force F_(ped). It hasactually been proved that in normal driving situations changes in thebrake pedal force of up to 200 N/s are practiced by the driver, whilstin danger situations the changes in the brake pedal force may reach 1500N/s or more. These values depend very much on the driver; in dangersituations some drivers may even practice changes in the brake pedalforce of up to 8000 N/s. Since it is more likely that the people with alower change in the brake pedal force do not brake sufficientlystrongly, it is expedient to equate the driver danger potential Δ_(da)for a change in the brake pedal force of 1500 N/s with 1 (Δ_(da)=1). Fora negative change in the brake pedal force the driver danger potentialΔ_(da) is equated with 0. Between the values 0 to 1500 N/s the driverdanger potential Δ_(da) increases from 0 to 1. For this transition acosine function is preferably selected because it shows continuoustransitions in the range of a change in the brake pedal force F_(ped)=0N/s and at F_(ped)=1500 N/s. Therefore the following formula applies toΔ_(da):$\Delta_{da} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{\max \left( {\frac{{\overset{.}{F}}_{ped}}{1500},0} \right)},1} \right)}} \right)}} \right\}}$

The development of this function is shown in FIG. 1.

The driving situation danger potential Δ_(ds), on the other hand isestimated on the basis of some parameters describing the drivingsituation such as the vehicle speed v₀ of the vehicle to be braked, thevehicle acceleration a₀, the distance d₀ towards the vehicle drivingahead, the relative speed {dot over (d)}₀ in relation to the vehicledriving ahead and the relative acceleration {umlaut over (d)}₀ inrelation to the vehicle driving ahead (see FIG. 2). This listing of theparameters is not concluding but additional parameters may be taken intoconsideration such as the wheelslip information that can be determinedby an intelligent brake system.

Out of the distance d₀ and its differentiations in time a theoreticalmoment of collision t_(c) can be determined after which both vehicleswould collide under constant speed and constant acceleration.

For d₀<0 and {umlaut over (d)}₀=0 the following applies:$t_{c} = {- \frac{d_{0}}{{\overset{.}{d}}_{0}}}$

If {umlaut over (d)}₀≠0, the following applies:$t_{c} = {- \frac{{- {\overset{.}{d}}_{0}} - \sqrt{{\overset{.}{d}}_{0}^{2} - {2d_{0}{\overset{¨}{d}}_{0}}}}{{\overset{¨}{d}}_{0}}}$

No collision occurs if t_(c) should be negative or complex and t_(c) isset to infinite. If, however, a collision would occur at constant speedand constant acceleration, the vehicle must be decelerated. At the sametime the reaction time t_(R) shall be taken into consideration.

The reaction time t_(R) has been statistically exactly determined and onaverage it amounts to 1.34 s; it consists of the following components:

Recognizing the danger situation and the object, respectively (0.48 s),

basic reaction time whilst neuronal processes trigger the correspondingphysical reactions (0.45 s),

movement time interval for moving the foot from the accelerator to thebrake pedal (0.19 s),

building up the brake pressure from the moment of touching the brakepedal (0.05 s)

and transmission of the brake pressure to the individual brake elements(0.17 s).

If t_(c)<t_(R) applies, an accident can no longer be avoided. Ift_(c)≧t_(R) applies, an acceleration a_(cva) is calculated for thevehicle to be braked showing the possible maximum acceleration (or theminimum required deceleration) for the avoidance of the accident. Thispossible maximum acceleration a_(cva) can be calculated with thefollowing formula:$a_{cva} = {{- \frac{1}{2}}\frac{\left( {v_{0} + {a_{cv0}t_{R}}} \right)^{2}}{d_{0} - {v_{0}t_{R}} - {\frac{1}{2}a_{cv0}t_{R}^{2}} - {\frac{1}{2}\frac{\left( {v_{0} + {\overset{.}{d}}_{0}} \right)^{2}}{a_{cv0} + {\overset{¨}{d}}_{0}}}}}$

The calculated acceleration is the permitted maximum acceleration afterthe reaction time t_(R). The driver, however, can already brake with theactual acceleration a_(cvo) so that no additional deceleration isrequired. If one would then determine the danger potential solely on thebasis of the permitted maximum acceleration, it could result in anunnecessary additional deceleration. It is therefore expedient to usethe difference a_(cve) between the permitted maximum accelerationa_(cva) and the actual acceleration a_(cvo) as the basis for determiningthe danger potential Δ (a_(cve)=a_(cva)−a_(cvo)). This is the additionaldeceleration the driver has to achieve after the end of the reactiontime t_(R). Since a deceleration higher than 0.9 g as a rule cannot beachieved, Δ=1 is set for a_(cve)=0.9 g.

For a_(cve)≧0 the risk of an accident is low, so that Δ=0 can be set fora_(cve)≧0. This means that in case of an acceleration of the vehicle of,for example, 0.2 g, Δ would equal 1 if the necessary deceleration is 0.7g because then a_(cve) would amount to −0.9 g.

Decelerations between 0 to 0.3 g are customary in daily traffic and canbe easily achieved. In this range the danger potential Δ should notdeviate strongly from 0. Decelerations in the range of 0.6 g to 0.9 gare more infrequent. Therefore the danger potential Δ should amount toapproximately 1 in this range. In the range of between 0.3 g and 0.6 g Δshould rise from 0 to 1. A function complying with these requirements isshown in FIG. 3 and provided by the following formula:${\Delta_{acve}\left( a_{cve} \right)} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{- {\min \left( {\frac{a_{cve}}{0,{9 \cdot g}},0} \right)}},1} \right)}} \right)}} \right\}}$

For a_(cva)=−0.9 g and a_(cvo)=−0.7 g Δ_(acve) states a low dangerpotential. Since, however, a deceleration of 0.9 g is a very highdeceleration being close to the physical limits of the frictional forcebetween the wheels and the road, there is a considerable accident risk.Any minor change in the road conditions or the movement of the vehiclemay require a deceleration that is higher than the decelerationphysically possible. For this reason the vehicle should be quicklytransferred into a driving condition where a lower deceleration isrequired. Therefore it is expedient that the danger potential Δ is alsodirectly dependent upon a_(cva). A danger potential Δ_(acva) (FIG. 4),for example, is set equal to 1 for the range of a possible maximumdeceleration a_(m) and equal to 0 for a deceleration a₀ that is 0.2 glower than a_(m).${\Delta_{acva}\left( a_{cva} \right)} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {{\pi \cdot {\min \left( {\left( \frac{a_{cva} - a_{0}}{a_{0} - a_{m}} \right),0} \right)}},1} \right)}} \right\}}$

The entire danger potential Δ should be at least as high as the maximumof Δ_(acva) and Δ_(acve). If Δ_(acva) and Δ_(acve) are higher than 0,the actual danger is greater than what the individual values expressbecause they are based on two different sources of danger, sinceΔ_(acva) shows a limited adhesion between the road and the wheel for therequired acceleration whilst on the other hand Δ_(acve) states that thenecessary deceleration cannot be achieved. Therefore the total fromΔ_(acve) and Δ_(acva) is preferably used as the driving situation dangerpotential Δ_(ds) if a_(cve) is lower than 0:

Δ_(ds)=(a _(cve) ,a _(cva))=min(1,Δ_(acve)+Δ_(acva))

In order to avoid a jerky change in the braking torque and in order toround off the transitions at a_(acve) equal to 0 for a_(cva) lower thana₀, Δ_(ds) is defined for a_(acve)<0 and a_(acva)<a₀ as follows: withΔ_(ds)(a_(cve), a_(cva)) = Δ_(e) ⋅ Δ_(a)$\Delta_{e} = {{\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{- {\min \left( {\frac{a_{cve} - \left( {a_{0} - a_{m}} \right)}{a_{0} - a_{m}},0} \right)}},1} \right)}} \right)}} \right\}}\quad {and}}$$\Delta_{a} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{- {\min \left( {\frac{a_{cvu} - a_{1}}{a_{1} - a_{m}},0} \right)}},1} \right)}} \right)}} \right\}}$$a_{1} = {a_{m} + \sqrt{\left( {a_{0} - a_{m}} \right)^{2} - a_{cve}^{2}}}$

Δ_(ds) is thus a function of a_(acve) and a_(acva) (FIG. 6) both of themlying in the range from −1 g to 0.5 g.

With the danger potentials Δ_(ds) and Δ_(da), respectively, thusobtained the deceleration during a braking action can be adjusted to theactual danger conditions. The braking behavior of the brake, however,should be so adjusted that it is always foreseeable for the driver andhe/she has the feeling of having his/her brake under complete control atany time. Therefore the brake pressure should only be increased when thebrake pedal is activated. For this reason the brake pressure should notbe automatically increased when the brake pedal is standing still evenif the danger potential Δ is equal to 1.

With the help of the block schematic diagram shown in FIG. 7 and thefunction sequence shown in FIG. 8 a first embodiment example of a staticbrake adaption in accordance with the invention is explained below thatfulfills the aforementioned stipulated targets.

To begin with a driver danger potential Δ_(da) is calculated in theabove manner from the change in the pedal force F_(d). From the driverdanger potential Δ_(da) a set-point amplification factor Λ_(ref) iscalculated. If the danger potential Δ_(da) is lower than a thresholdvalue of, for example, 0.25, the set-point amplification factor Λ_(ref)is lower than the standard amplification factor Λ₀ of the brake servounit during the mechanical operation of its control bar. In this rangebelow the threshold value the braking action is exclusively carried outby the mechanical operation of the control bar.

If the danger potential exceeds the threshold value, the automatic brakeadaption starts as soon as the brake is activated; if at the same timedanger potential Δ increases the set-point amplification factor Λ_(ref)is increased proportionately to the danger potential Δ. It is expedientif the adapted set-point amplification factor Λ_(ref) corresponds to thestandard amplification factor Λ₀ in the range of the threshold valueΛ_(SCH), so that in case of a low danger potential there is not toostrong a change of the amplification factor Λ but an automatic change ofthe amplification factor Λ adapted to the particular danger situation,creating a braking behavior the driver can control well.

As shown in FIG. 7 a nominal pressure P_(ref) is determined from theset-point amplification factor Λ_(ref), the pedal force F_(ped), and thepressure present in the brake servo unit P_(TMC) to which the pressureof the brake servo unit P_(TMC) is set. Taking the position of the pedaland the corresponding pedal force F_(ped), respectively, intoconsideration guarantees that the pressure in the brake servo unit isonly increased if a corresponding control signal is created at the brakepedal.

Since the braking properties should not change during a braking action,they should at least not be reduced, a set-point amplification factorΛ_(ref) once set is not reduced in a preferred development form untilthe activation of the brake is discontinued, i.e. the foot is taken offthe brake pedal.

FIG. 9 shows an additional block schematic diagram of an embodimentexample carrying out a static brake adaption. Here the danger potentialΔ is determined according to the driving situation potential Δ_(ds),described above. If the danger potential is higher than a predeterminedinlet pressure threshold value of, for example, 0.4 and the permittedmaximum acceleration a_(cva) lower than 0, an inlet pressure functionwill be activated setting the nominal pressure P_(ref) to a certaininlet pressure P_(Preload) rated so high that a braking pressure isbuilding up in the brake pipes that is just not sufficient to activatethe brakes. As a result the pressure in the brake pipes does not have tobuild up during an activation of the brakes to be expected later and forthis reason the brakes will respond faster in such a danger situation.

The nominal pressure P_(ref) is determined in the same way as in theembodiment example described on the basis of FIG. 7; in the case of anactive inlet pressure function the higher of the two determined nominalpressures P_(ref) is used as the nominal pressure P_(ref) for triggeringthe brake servo unit.

If the danger potential Δ is higher than a previously defined warningsignal threshold value that has been set to 0.5 in the presentembodiment example, a warning signal is issued in the passengercompartment warning the driver about the danger situation.

If the danger potential Δ exceeds any traffic warning signal thresholdvalue that is preferably higher than the warning signal threshold valueand in the present embodiment example is set to 0.7, the brake lightsare switched on to warn the following vehicles. In order to distinguishit from an ordinary braking the brake lights can be switched on blinkingor a different warning signal can be issued so that the followingtraffic is clearly aware of the danger situation and a correspondingsafe distance can be maintained.

Thus the determination of the danger potential can be used incombination with the brake adaption, but also independent of it, to warnthe driver and/or traffic by creating a preventive measure for theavoidance of accidents.

On the basis of FIG. 10 an additional embodiment example of theinvention with a dynamic brake adaption is explained.

The determination of the danger potential Δ_(ds) is carried out in theway described above. However, the set-point amplification factor Λ_(ref)is not calculated out of the danger potential, but the nominal pressureP_(ref) is directly calculated in dependence of a change (=1stdifferentiation in time) of the brake pedal position, the brake pressureor such, so that with the increasing brake pedal pressure the nominalpressure P_(ref) is dynamically increasing. Here it is essential that inorder to determine the nominal pressure P_(ref) the firstdifferentiation in time of a physical variable is used that is connectedwith the activation of the pedal by the driver so that a change in theactivation of the pedal results in a dynamic increase or decrease of thebrake pressure.

FIG. 11 shows an additional embodiment example of the invention with adynamic brake adaption that is equipped with the additional functionsdescribed above for issuing a warning signal and switching on the brakelights, respectively.

In case of the embodiment examples described above the danger potentialis determined either on the basis of the driver danger potential Δ_(da)or the driving situation danger potential Λ. It goes without saying thatthe danger potential can also be determined on the basis of acombination of the driver danger potential Δ_(da) and the drivingsituation danger potential Λ_(ds).

The success of the invention is essentially based upon the evaluation ofcurrent physical parameters in combination with statistical values drawnfrom past experience allowing a meaningful estimate of the dangerpotential. Therefore the invention can be modified by including variousstatistical results, in particular the evaluation of accident statisticssaved in an electronic storage.

Similar to the characteristic diagrams known from the ignition devicesthese accident statistics are retrieved in dependence of predeterminedparameters such as speed, acceleration, activation of the brake pedal,etc.

What is claimed is:
 1. Process for shortening the braking distance of avehicle equipped with a brake servo unit which during a standard brakingaction is only triggered by the brake pedal pressure caused by thedriver and in case of a critical driving situation is so triggered by acontrol device that in comparison to the standard braking action anincreased amplification factor is set at the brake servo unit,comprising the steps of: determining a danger potential by defining theprobability that the vehicle to be braked will be involved in anaccident, and increasing the amplification factor in accordance with thedanger potential, wherein the amplification factor is only increased bythe control device above a certain threshold value of the dangerpotential, wherein the amplification factor increased by the controldevice within the range of the threshold value corresponds essentiallyto the standard amplification factor available during the standardbraking action and is continuously increased as the danger potentialincreases, wherein the danger potential is determined on the basis ofone or several parameters influenced by the driver (driver dangerpotential Λ_(da)) or one or several parameters induced by a drivingsituation (driving situation danger potential Λ_(ds)), wherein thedanger potential (Λ_(da)) is calculated according to the followingformula:$\Delta = {{\cos \left( \frac{{\overset{.}{F}}_{ped}}{{\overset{.}{F}}_{{ped} - {panic}}} \right)} \cdot}$


2. Process according to claim 1, wherein the danger potential parametersare selected from the set including: the speed used to remove a footfrom the accelerator, the pedal changing time required to change fromthe accelerator over to the brake pedal, the brake pedal travel, thebrake pedal speed, the diaphragm travel (s_(mem)) of a diaphragmprovided within the brake servo unit or its time differential, the brakepedal force, the change in the brake pedal force and/or the change inthe steering angle.
 3. Process according to claim 2, wherein acombination of at least two parameters is used to determine the dangerpotential.
 4. Process according to claim 3, wherein at least one of thedanger potential parameters is weighted with a weighting factor g_(i)according to the following formula:$\Delta_{da} = {\sum\limits_{i = 1}^{n}\quad {{{Parameter}_{l}} \cdot g_{i}}}$


5. Process according to claim 1, wherein the danger potential iscalculated according to the following formula:${\Delta_{da} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{\max \left( {\frac{{\overset{.}{F}}_{ped}}{1500},0} \right)},1} \right)}} \right)}} \right\}}},$

at the same time {dot over (F)}_(ped) represents the change in thebraking force.
 6. Process according to claim 1, wherein the dangerpotential is determined by using at least one of the followingparameters: the vehicle speed (v₀), the vehicle acceleration (a₀), thedistance (d₀) to a vehicle driving ahead, the relative speed ({dot over(d)}₀) in relation to a vehicle driving ahead, the relative acceleration({umlaut over (d)}₀) in relation to a vehicle driving ahead or wheelslipinformation.
 7. Process according to claim 6, wherein the dangerpotential is determined on the basis of a difference from a permittedmaximum acceleration and an actual acceleration.
 8. Process according toclaim 7, wherein the permitted maximum acceleration is calculatedaccording to the following formula:${a_{cva} = {{- \frac{1}{2}}\frac{\left( {v_{0} + {a_{cv0}t_{R}}} \right)^{2}}{d_{0} - {v_{0}t_{R}} - {\frac{1}{2}\quad a_{cv0}t_{R}^{2}} - {\frac{1}{2}\frac{\left( {v_{0} + {\overset{.}{d}}_{0}} \right)^{2}}{\quad {a_{cv0} + {\overset{¨}{d}}_{0}}}}}}},$

wherein, t_(R) is the reaction time, v₀ the vehicle speed and d₀, {dotover (d)}₀, {umlaut over (d)}₀ the distance to the vehicle driving aheadand its differentiations in time.
 9. Process according to claim 7,wherein the danger potential is calculated according to the followingformula:${\Delta_{acve}\left( a_{cve} \right)} = {\frac{1}{2} \cdot {\left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{- {\min \left( {\frac{a_{cve}}{0,{9 \cdot g}},0} \right)}},1} \right)}} \right)}} \right\}.}}$


10. Process according to claim 9, wherein the danger potential isdirectly dependent upon a permitted maximum speed.
 11. Process accordingto claim 10, wherein the danger potential is calculated according to thefollowing formula:${{\Delta_{acva}\left( a_{cva} \right)} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {{\pi \cdot {\min \left( {\left( \frac{a_{cva} - a_{0}}{a_{0} - a_{m}} \right),0} \right)}},1} \right)}} \right\}}},$

wherein, a_(cva) is the permitted maximum acceleration, a_(m) thepossible maximum deceleration, and a₀ is a declaration that is lowerthan a_(m) by at least 0.1 g.
 12. Process according to claim 1, whereinthe increase of the amplification factor is only carried out when thebrake pedal is activated.
 13. Process according to claim 12, wherein theamplification factor is carried out proportionally to the brake pedalforce or a variable corresponding to it.
 14. Process according to claim12, wherein the increase in the amplification factor is carried outproportionally to the change in the brake pedal force or a variablecorresponding to it.
 15. Process according to claim 14, wherein anincrease in the amplification factor, once taken, is not reduced untilthe activation of the brake is discontinued.
 16. Process according toclaim 15, wherein if the danger potential becomes higher than apredetermined inlet pressure threshold value and a permitted maximumspeed is lower than 0, then an inlet pressure function is activatedwhich sets a certain inlet pressure in the brake servo unit dimensionedsufficiently high such that a braking pressure is built up in the brakepipes that is insufficient to activate the brakes.
 17. Process accordingto claim 16, wherein when the danger potential is higher than apredetermined warning signal threshold value, a warning signal is issuedin the passenger compartment.
 18. Process for shortening the brakingdistance of a vehicle equipped with a brake servo unit which during astandard braking action is only triggered by the brake pedal pressurecaused by the driver and in case of a critical driving situation is sotriggered by a control device that in comparison to the standard brakingaction an increased amplification factor is set at the brake servo unit,comprising the steps of: determining a danger potential by defining theprobability that the vehicle to be braked will be involved in anaccident, and increasing the amplification factor in accordance with thedanger potential, wherein the amplification factor is only increased bythe control device above a certain threshold value of the dangerpotential, wherein the amplification factor increased by the controldevice within the range of the threshold value corresponds essentiallyto the standard amplification factor available during the standardbraking action and is continuously increased as the danger potentialincreases, wherein the danger potential is determined on the basis ofone or several parameters influenced by the driver (driver dangerpotential Λ_(da)) or one or several parameters induced by a drivingsituation (driving situation danger potential Λ_(ds)) wherein the dangerpotential is calculated according to the following formula:${\Delta_{da} = {\frac{1}{2} \cdot \left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{\max \left( {\frac{{\overset{.}{F}}_{ped}}{1500},0} \right)},1} \right)}} \right)}} \right\}}},$

 at the same time {dot over (F)}_(ped) represents the change in thebraking force.
 19. Process for shortening the braking distance of avehicle equipped with a brake servo unit which during a standard brakingaction is only triggered by the brake pedal pressure caused by thedriver and in case of a critical driving situation is so triggered by acontrol device that in comparison to the standard braking action anincreased amplification factor is set at the brake servo unit,comprising the steps of: determining a danger potential by defining theprobability that the vehicle to be braked will be involved in anaccident, and increasing the amplification factor in accordance with thedanger potential, wherein the amplification factor is only increased bythe control device above a certain threshold value of the dangerpotential, wherein the amplification factor increased by the controldevice within the range of the threshold value corresponds essentiallyto the standard amplification factor available during the standardbraking action and is continuously increased as the danger potentialincreases, wherein the danger potential is determined on the basis ofone or several parameters influenced by the driver (driver dangerpotential Λ_(da)) or one or several parameters induced by a drivingsituation (driving situation danger potential Λ_(ds)), wherein thedanger potential is determined by using at least one of the followingparameters: the vehicle speed (v₀), the vehicle acceleration (a₀), thedistance (d₀) to a vehicle driving ahead, the relative speed ({dot over(d)}₀) in relation to a vehicle driving ahead, the relative acceleration({umlaut over (d)}₀) in relation to a vehicle driving ahead or wheelslipinformation wherein the danger potential is determined on the basis of adifference from a permitted maximum acceleration and an actualacceleration wherein the permitted maximum acceleration is calculatedaccording to the following formula:${a_{cva} = {{- \frac{1}{2}}\frac{\left( {v_{0} + {a_{cv0}t_{R}}} \right)^{2}}{d_{0} - {v_{0}t_{R}} - {\frac{1}{2}\quad a_{cv0}t_{R}^{2}} - {\frac{1}{2}\frac{\left( {v_{0} + {\overset{.}{d}}_{0}} \right)^{2}}{\quad {a_{cv0} + {\overset{¨}{d}}_{0}}}}}}},$

 wherein, t_(R) is the reaction time, v₀ the vehicle speed and d₀, {dotover (d)}₀, {umlaut over (d)}₀ the distance to the vehicle driving aheadand its differentiations in time.
 20. Process for shortening the brakingdistance of a vehicle equipped with a brake servo unit which during astandard braking action is only triggered by the brake pedal pressurecaused by the driver and in case of a critical driving situation is sotriggered by a control device that in comparison to the standard brakingaction an increased amplification factor is set at the brake servo unit,comprising the steps of: determining a danger potential by defining theprobability that the vehicle to be braked will be involved in anaccident, and increasing the amplification factor in accordance with thedanger potential, wherein the amplification factor is only increased bythe control device above a certain threshold value of the dangerpotential, wherein the amplification factor increased by the controldevice within the range of the threshold value corresponds essentiallyto the standard amplification factor available during the standardbraking action and is continuously increased as the danger potentialincreases, wherein the danger potential is determined on the basis ofone or several parameters influenced by the driver (driver dangerpotential Λ_(da)) or one or several parameters induced by a drivingsituation (driving situation danger potential Λ_(ds)) wherein the dangerpotential is determined by using at least one of the followingparameters: the vehicle speed (v₀), the vehicle acceleration (a₀), thedistance (d₀) to a vehicle driving ahead, the relative speed ({dot over(d)}₀) in relation to a vehicle driving ahead, the relative acceleration({umlaut over (d)}₀) in relation to a vehicle driving ahead or wheelslipinformation wherein the danger potential is determined on the basis of adifference from a permitted maximum acceleration and an actualacceleration wherein the danger potential is calculated according to thefollowing formula:${\Delta_{acve}\left( a_{cve} \right)} = {\frac{1}{2} \cdot {\left\{ {1 - {\cos \left( {\pi \cdot {\min \left( {{- {\min \left( {\frac{a_{cve}}{0,{9 \cdot g}},0} \right)}},1} \right)}} \right)}} \right\}.}}$